Both the manufacturer and the user of a machine part generally have serious concerns as to whether that part is truly made of a material to specifications. This is because the use of an incorrect or defective material has high potential to lead directly to a disastrous accident due to failure of the part.
Therefore, the manufacturer makes it a rule to conduct materials tests on each of the parts produced, and guarantees the authenticity of an individual part by issuing a materials test performance list which certifies that it has been produced from the material to specifications. In the case of cast iron products, the materials tests to be conducted include a tensile test and a hardness test. As for spheroidal graphite iron products, an additional test is conducted to determine the percent spheroidicity of graphite.
While this is generally the basic way adopted to certify the authenticity of materials, materials tests require about three or four days including the processing of test pieces and other steps, so the manufacturer has desired the development of a method that enables various materials characteristics to be estimated in a simpler and quicker way.
On the side of the user, a need exists for verifying on the actual sample that the part of interest is truly made of the material to specifications. However, to prepare a test piece for materials testing, the actual sample must be broken, and it has been desired to develop a method by which tensile strength and other properties can satisfactorily be estimated in a nondestructive manner.
As for the test to estimate the percent spheroidicity of graphite in spheroidal cast irons to be evaluated by various materials tests, an apparatus for ultrasonic measurement was developed in the early eighties that was capable of indirect determination of the percent spheroidicity of graphite using the fact that the velocity of sound (the term "sound velocity" as used herein means the speed of propagation of ultrasonic waves) varied with the shape of graphite particles.
A block diagram of an apparatus that measures the sound velocity on a cast specimen by ultrasonic wave and which computes automatically the percent spheroidicity of graphite in the specimen is shown in FIG. 16.
In the drawing, numeral 1 designates an ultrasonic probe, 2 is an ultrasonic flaw detector, 3 is a D/A converter circuit, 4 is a bus line, 5 is a ROM, 6 is a RAM, 7 is a keyboard (KBD), 8 is a CRT, and 9 is a microprocessor (MPU). Details of interface circuits and the like that are connected between these components are omitted from FIG. 16.
Further referring to FIG. 16, numeral 51 designates a program for measuring the sound velocity, 52 is a program for computing the percent spheroidicity, 53p is a V-S conversion formula, and 54p is a main program; these programs are stored in ROM 5 and executed by MPU 9 to perform the functions they are assigned respectively.
Sound velocity measuring program 51 is activated by main program 54p when it is instructed to start measurement via keyboard 7, and the ultrasonic flaw detector 2 is controlled via bus line 4 to measure the sound velocity on the specimen 1a. Stated more specifically, the ultrasonic wave sent from the ultrasonic probe 1 is partly reflected by the surface of the specimen 1a, whereas the remainder propagates through the interior of the specimen 1a and is also reflected by its bottom. These reflected waves are detected with the ultrasonic probe 1 and upon receiving the detection signal, the ultrasonic flaw detector 2 measures the time from the point of detection of the surface reflected wave to the point of detection of the bottom reflected wave. The measured time is sent to the D/A converter circuit 3 and thence delivered to MPU 9 as a digital value. The delivered time is the time taken by the ultrasonic wave to go back and forth through the specimen 1a and, hence, the input digital value is divided by twice the thickness of the specimen 1a that is preliminarily measured and which has already been entered from the keyboard 7 and, as a result of this operation, the sound velocity on the specimen 1a is determined.
The thus-measured value of sound velocity is stored in area V in RAM 6 by means of the sound velocity measuring program 51.
The program 52 for computing the percent spheroidicity is subsequently activated by the main program 54p and performs a conversion process in accordance with the V-S conversion formula 53p, thereby computing the percent spheroidicity of graphite from the value of sound velocity stored in area V. The computed percent spheroidicity of graphite is stored in area S in RAM 6.
The V-S conversion formula 53p represents the relationship between the sound velocity and the percent spheroidicity of graphite for castings, and is an empirically determined conversion formula. Stated specifically, this is a regression line as constructed by plotting the results of measurement on a plurality of castings in relevant positions on a coordinate system, the horizontal axis of which may typically represent the sound velocity (V) as determined by ultrasonic measurement while the vertical axis represents the percent spheroidicity of graphite (S) as determined by direct means of measurement in accordance with the JIS (Japanese Industrial Standards) (see 53a in FIG. 17).
The percent spheroidicity of graphite stored in area S which has been computed on the basis of this empirical formula is displayed on CRT 8 by the main program 54p as the result of measurement.
The tensile strength of castings is measured directly by a so-called "tensile test", or a test in which the load or the like that is applied when a test piece worked to a prescribed shape breaks is measured on a calibrated tensile tester.
However, the tensile test takes time in preparing test pieces, working them, conducting the test, etc. and, hence, three or four days are necessary before the inspector knows the acceptability of the material being tested. Under the circumstances, there has been a need for a method by which the tensile strength of a cast product can be known as soon as it is produced, and which enables one to be sure that the cast product has been definitely yielded from the material to specifications.
The tensile strength of a cast product depends on the tensile strength of its base and the shape of fine graphite grains that are distributed in the base and, hence, the tensile strength of the cast product would be estimated by combining its hardness, which is a substitute value of the tensile strength of the base, with the percent spheroidicity of graphite which indicates the shape of graphite. On the basis of this idea, a method has been proposed that determines indirectly the tensile strength of the cast product from the value of sound velocity and hardness.
FIG. 20 is a block diagram showing an apparatus for implementing this indirect method, namely, an apparatus for ultrasonic measurement that measures the sound velocity on a cast specimen by ultrasonic wave and which computes automatically the tensile strength of the specimen from the measured value of sound velocity and the value of hardness as measured in a separate step. In FIG. 20, the constituent elements that are the same as those which are shown in FIG. 16 are identified by like numerals.
In the drawing, numeral 53 refers to a classification program, 54 is a tensile strength computing program, and 55 is a main program, all of these programs being new.
These programs are stored in ROM 5 and executed by MPU 9 to perform the functions they are assigned respectively.
The operation of the apparatus shown in FIG. 20 is described below without going into details of the part of the operation that has already been explained in connection with the case shown in FIG. 16. First, the sound velocity on the specimen 1a is measured and stored in area V in RAM 6. Then, the percent spheroidicity of graphite is computed from the stored value of sound velocity in area V in accordance with the empirical V-S conversion formula, and then stored in area S in RAM 6. Stated specifically, this V-S conversion formula is a regression line as is constructed by plotting the results of measurement on a plurality of castings in relevant positions on a coordinate system, the horizontal axis of which may typically represent the sound velocity (V) as determined by ultrasonic measurement while the vertical axis represents the percent spheroidicity of graphite (S) as determined by direct means of measurement in accordance with the JIS (see 53a in FIG. 17).
When the percent spheroidicity of graphite (S) is determined, the classification program 53 is activated by the main program 55. Then, the state of castings structure is classified by the classification program 53 in accordance with the value of the percent spheroidicity of graphite (S), and a value indicating the specific type, such as gray cast iron (FC), CV graphite cast iron (FCV), or spheroidal graphite cast iron (FCD), is written into area (F). For the sake of reference, the classification according to the JIS specifications is shown in FIG. 18.
In the next step, the Brinell hardness of the specimen 1a as measured with a separate hardness tester is entered via the keyboard 7 and stored in area HB. Then, the tensile strength computing program 54 is activated by the main program 55. In accordance with the formula for conversion from the product of sound velocity and hardness to tensile strength, the program 54 computes the tensile strength from the product of sound velocity (V) and Brinell hardness (HB) that have been measured on the specimen 1a. The computed tensile strength is stored in area .sigma.B' in RAM 6.
It should be noted here that the above-mentioned formula for conversion to tensile strength consists of three expressions that are selectively used depending upon the state of castings structure: conversion expression 54a for FCD, conversion expression 54b for FCV, and conversion expression 54c for FC. Stated specifically, these expressions are regression lines as constructed by plotting the results of direct measurement on a plurality of castings in relevant positions on a coordinate system, the horizontal axis of which represents the product of sound velocity and hardness (V.times.HB) while the vertical axis represents the tensile strength (.sigma.B) (see 54a, 54b, and 54c in FIG. 19).
The thusly determined tensile strength (.sigma.B') and the like are displayed on CRT 8 by the main program 55 as the result of measurement.
In the case of spheroidal graphite cast iron and CV graphite cast iron, it is also necessary to know their percent elongation as a characteristic value for verifying that they are made of the material to specifications.
The percent elongation of castings has heretofore been measured by a method that depends on a tensile test as conducted using test pieces having a prescribed specification geometry.
However, no convenient and indirect substitute method has ever been developed. As already mentioned, the material species of cast iron can be specified by the materials characteristic values that are attained by materials tests including a tensile test and the like; among them, the percent spheroidicity of graphite and the tensile strength have been the subject of reviews on proposed alternatives that rely upon the technique of measuring the sound velocity. However, sound velocity measurement using an apparatus for ultrasonic measurement involves variations in the result of measurement due to the apparatus such as those in the characteristics of the ultrasonic probe, the ultrasonic flaw detector and D/A conversion. Further, one cannot neglect the variations in the result of measurement due to the inspector such as those in the measurement of thickness of the specimen and in adjustments like the setting of the gate to the ultrasonic flaw detector.
Under these circumstances, the empirical formula on the percent spheroidicity of graphite that has been attained by a certain apparatus and inspector for ultrasonic measurement (see 53a in FIG. 14) does not necessarily agree with the empirical formula that has been attained by another apparatus and inspector for ultrasonic measurement (see 53b in FIG. 15). The sound velocity that should correspond to a cast product having 70% spheroidicity of graphite is 5.56 km/s in one case but it is 5.62 km/s in the other case, and the two values differ considerably. What is more, such differences occur frequently. Therefore, in spite of the nondestructive and convenient nature of the measurement, the result is not reliable and this causes problems.
Next, the specific problems as regards the measurement of the percent spheroidicity of graphite will be further discussed. In the conventional method and apparatus for ultrasonic measurement, an approximate value of percent spheroidicity of graphite is computed from the sound velocity in accordance with an available empirical formula. By means of this indirect technique, the state of castings structure is analyzed in a nondestructive manner.
However, the relationship between the sound velocity and the percent spheroidicity of graphite is such that one does not correlate well to the other (see FIG. 17). Hence, an approximate value of the percent spheroidicity of graphite that has been attained by the empirical formula at issue or the result of analysis on the state of castings structure contains such large errors that they are by no means suitable for practical use as substitutes for the true percent spheroidicity of graphite.
Under the circumstances, inspections such as nondestructive delivery or acceptance inspection of cast products and the like that have damage have only low reliability in the results or inspection and, even if certain troubles occur in the production process, one may often overlook the adverse effects of such troubles by failing to detect them.
Similar problems occur in connection with the measurement of tensile strength on castings. A plurality of formulae exist for conversion to tensile strength (see FIG. 19) which are derived on the basis of the results of measurement on the typical states of casting structures that belong to the respective types. Since no general-purpose expression, relation, or the like that unifies them has yet been established, these formulae are selectively used in accordance with the specific type of casting. Under the circumstances, a cast product of an intermediate state that is not typical is dealt with by different conversion formulae depending upon the type in which the cast product is classified, and this causes great variations in the computed result of measurement; hence, the conversion formulae under consideration are by no means suitable for practical use as means of warranting the product and the like.
There are also problems in connection with the measurement of percent elongation on castings. These are drawbacks including the following: In the state of the art, the only method that can be adopted is by conducting a tensile test and, since this involves a destructive measurement, not all products in the production lot can be inspected; since the geometry of test pieces is prescribed by specifications, several days of time and cost are taken in preparation for the measurement; and, since a dedicated tensile test apparatus is necessary, no inspectors other than testing organizations and the like which have suitable apparatus are able to conduct the measurement unless they ask for help by an outsider.
An object, therefore, of the present invention is to solve these problems of the prior art by realizing a method and apparatus for ultrasonic measurement on castings by which the values of state analysis and characteristic values (e.g. the percent spheroidicity of graphite, state classification, tensile strength, and percent elongation) of cast specimens can be measured or computed with high probability in a nondestructive and convenient manner.